Alumina trihydrate (also referred to as aluminum hydroxide, ATH, alumina, hydrated alumina, and aluminum trihydroxide) is a white crystalline powder used as a flame retardant and smoke suppressant in a number of applications including polymeric wire and cable compositions. When heated, ATH absorbs heat and releases water. This lowers the surface temperature of the polymeric composition to which the ATH is added which provides a barrier to combustion and dilutes the gases of combustion.
ATH occurs naturally as the mineral gibbsite or hydragyllite, or it is obtained directly from bauxite by the Bayer process which involves the treatment of bauxite with alkali under pressure. A common type of ATH utilized in elastomeric or polymeric compositions is an ultrafine precipitated grade ATH (PATH). PATH has been known to undesirably raise the level of the viscosity of the composition to which it is added. In order to minimize the increase in the viscosity of the composition, it is known to treat the PATH with a silane. Some commonly used silanes used in the treatment of ATH are described in U.S. Pat. Nos. 4,526,922 and 4,550,056. While the silane treatment of PATH promotes lower compound viscosity, the PATH particles still yield relatively high viscosities when added to elastomeric or polymeric formulations.
Ground ATH (GATH) has certain advantages over precipitated materials. For example, GATH filled polymeric composites typically yield lower compound viscosities and improved processing. Further, the surface area of the particles of precipitated ATH products are typically lower than the surface area of the particles of ground ATH products of the same average particle size. Higher surface area provides better flame retardancy and smoke suppression in polymeric formulations. However, this higher surface area of the ground ATH particles, particularly in vinyl silane coupled, peroxide or radiation cured compounds, has a negative impact on elongation at break. The low elongation has prevented GATH grades from replacing PATH grades in many applications such as wire and cable insulation and jacketing compounds.